PROJECT SUMMARY Human babesiosis is a malaria-like multisystem disease caused primarily by Babesia microti, an emerging apicomplexan parasite that infects and develops within human erythrocytes. The parasite is transmitted to humans by the tick vector Ixodes scapularis and can also be introduced through blood transfusion. Infection can cause flu-like symptoms, and severe infection can be fatal, in particular in the immunosuppressed and the elderly. Current methods for babesiosis diagnosis include microscopy, PCR, IFA and ELISA-based methods that detect antibodies in serum from patients or donors. Each of these methods has major limitations, such as insufficient sensitivity, high complexity, and low throughput. In addition, results from many of these assays remain positive for months or years after resolution of active infection. In Phase I of this project, we have developed a capture ELISA assay that can detect a protein of the parasite, BmGPI12/BmSAI. We have identified pairs of monoclonal antibodies that can be used to capture BmGPI12 in mouse and human samples. Our preliminary results demonstrate that this assay has an excellent sensitivity. Using samples collected from infected mice, a well-established model of human disease, we demonstrated that following successful treatment, our assay gives negative results while PCR and serology remain positive. Thus, our assay may be useful in distinguishing active infection from past exposure. In Phase II of this project, we will further develop this assay with the goal of applying for FDA clearance. We will analyze several hundred animal and human samples to establish important characteristics of the assay including sensitivity and specificity. These Phase II experiments are designed to give us a full understanding of the potential biological indications and commercial usefulness of the assay.